Open Systems Interconnection (OSI) layers are known to those of skill in the art as a series of protocol layers to define communications in data networks. The first layer relates to the physical aspects of communication. Examples are T-1 and 100-base T. The second layer is called the data link layer. This layer is used to format data passing over a given link. Examples include Ethernet and HDLC. Layer 3 is called the network layer. This layer supports end-to-end packet delivery and the most common example is the IP routing in the Internet. Layer 4, the transport layer, provides end-to-end management of communications.
Networks that use a connection as the primary method of transporting information between two points are considered Layer 4 networks as Layer 4 protocols such as TCP can mange these networks directly.
Telephone networks can be considered Layer 4 as a connection must be made before any communication can occur. The earliest of these automatic telephone networks used Strowger switches and were called “direct control” or “distributed control” switching. Fundamental characteristics of distributed control are that each path is built through the network independently. The establishment of a connection is performed on a switch-by-switch basis. Once established, the connection is held up through the switch train by each end.
As telephone networks became more complex, distributed control was replaced by common control switching, and continues to this day. Common control switching is characterized by the use of computers with knowledge of the network that establish connections or route packets.
The PSTN today uses SS7, a common control switching system, to switch calls as multiple, complex routing decisions must be made for each call.
Our first data networks used telephone switches to establish connections between computers. ISDN is a technology developed by TELCOS to switch both voice and data. It soon became evident that the delay and overhead included in making connections were unacceptable to the computer world, and ISDN was replaced by packet networks.
Packet networks such as the internet are said to be OSI Layer 3 networks, as the network is connectionless and each packet is routed on a “best effort” basis.
When computers today wish to transmit packet flows reliably they use Layer 4 protocols such as TCP to provide packet ordering and retransmission. TCP establishes and manages a virtual connection over a Layer 3 packet network.
As Layer 3 networks are connectionless and route data on a best effort basis, routes can often become congested. Congestion can harm time-sensitive traffic such as voice or video as the delay or packet loss associated with congestion will render the connection unusable.
Companies are developing improvements to the packet network such as quality of service (QOS) to allow priority traffic such as voice or video to not be affected by congestion. Some companies have even developed a concept called “flow-based routing” whereby packets are organized into flows, with each flow being routed or switched separately. These systems tend to be complex and expensive as specialized hardware and software are needed to find the Layer 4 information relating to the flows imbedded in the Layer 3 packets.
Because of the aforementioned problems, label-switching technologies such as frame relay, ATM, and MPLS have become popular for OSI layer 2 wide area networking. The short labels are popular with telecommunication carriers as a more efficient alternative to traditional IP routing. The most popular of these technologies, multi-protocol label searching (MPLS), uses label switched paths “LSPs” to carry packet flows between edge nodes. Packets in these flows are transported in a deterministic, orderly manner. In fact, transport schemes of this nature are so reliable that the term “pseudowire” has been used to describe this system. Through the use of MPLS, packet flows through LSPs have been used to interconnect LANS (VLANS), support QOS and policy routing, and even switch synchronous services such as DS1s or DS3s.